This invention relates to aqueous copolymer compositions and in particular to self-stabilized aqueous copolymer emulsions. The aqueous copolymer compositions are stabilized by block and/or graft copolymers derived from the copolymerization of a water soluble or dispersible macromonomer with a monomer or blend which is water insoluble.
Conventional emulsion copolymerization is a process in which a copolymer is formed directly in water using surfactants (i.e., compounds which are able to form micelles in the aqueous phase), which stabilize the copolymer particles in the emulsion. Surfactants may be non-ionic (e.g., alkyl or alkylphenol ethoxylated derivatives); anionic (e.g., salts of alkyl sulfates, phosphates or sulfonates); or cationic (e.g., quaternary ammonium salts of alkyl amines). Using such surfactants, which remain in the free form as a water soluble species in the emulsion, can cause problems when the emulsions are used as coating compositions, such as poor humidity and corrosion performance. It would therefore be an advantage if such copolymer emulsions could be prepared without the use of the monomeric, water sensitive surfactants.
U.S. Pat. No. 5,936,026 is directed to surfactant-free emulsions and coating compositions containing same. The emulsion is prepared by emulsion copolymerization of a monomer blend in the presence of a water soluble or water dispersible macromonomer with at least 10% of unsaturated monomers all having acid or all having amine groups in an ionized form. At least 2% and preferably 10-40% of macromonomer is needed to prepare stable emulsions, based on the overall ionized acid or amine content in the macromonomer. Such copolymer emulsions do still have a high overall level of water sensitive groups. The macromonomers do not form micelles like conventional surfactants, which is why the higher levels of macromonomer are needed to sufficiently stabilize the overall copolymer emulsion.
U.S. Pat. No. 5,362,826 discloses a method of preparing macromonomers from oligomers with a terminal unsaturated end-group. The oligomers are prepared by cobalt catalytic chain transfer polymerization as described in U.S. Pat. Nos. 4,680,352 and 4,722,984. There is no teaching in those references that the macromonomers can be used as surfactants in an emulsion copolymerization process. PCT/US95/00376 teaches aqueous branched copolymers having hydrophobic macromonomer arms and a hydrophilic backbone. These copolymers are prepared in solvent and then inverted into water. There is no teaching or suggestion that such copolymers are suitable for use in emulsion coatings.
U.S. Pat. No. 5,231,131 relates to pigment dispersions in an aqueous carrier containing a graft copolymer having a polymeric backbone which is hydrophobic in nature as compared to the side chains which are hydrophilic. The side chains consists of hydrophilic macromonomers with 20-50% of polymerized acid functional co-monomer which are neutralized with an amine or inorganic base. There is no teaching of using such graft copolymers as surfactants in an emulsion copolymerization.
An emulsion useful for forming coating compositions comprising
a) an aqueous carrier;
b) a polymer mixture comprising:
(1) 40-99.5% by weight of a copolymer with a weight average molecular weight of at least 3000; and
(2) 0.5-60% by weight of a stabilizing copolymer dispersed in water, said stabilizing copolymer comprising a hydrophobic portion and a hydrophilic portion, wherein said stabilizing polymer is selected from the group consisting of:
(i) polymers comprising 5-95% by weight of a hydrophilic macromonomer having at least 10% by weight of an acid functional monomer and 5-90% by weight of at least one hydrophobic monomer polymerized in the presence of the macromonomer;
(ii) polymers comprising 5-95% by weight of a hydrophobic macromonomer and 5-95% by weight of hydrophilic copolymer comprising at least 10% by weight of an acid functional monomer polymerized in the presence of the macromonomer.
The stabilizing copolymer can comprise a graft copolymer having a hydrophobic backbone with hydrophilic macromonomer arms, a hydrophilic backbone with hydrophobic macromonomer arms, or can comprise an AB block copolymer having a hydrophobic or hydrophilic macromonomer as the A block and a hydrophilic or hydrophobic copolymer as the B block.
The macromonomers preferably comprise the polymerization product of at least 50% by weight of monomers selected from methacrylate, methacrylonitrile, methacrylamide and derivatives and mixtures thereof with a weight average molecular weight of 500 to 10000 and prepared by cobalt catalytic chain transfer polymerization.
The process for making the emulsion and coating compositions based on the emulsion are also part of this invention.
In a most preferred embodiment, the acid value of the emulsion is less than 32.
Emulsion copolymerization is a process of forming a copolymer from ethylenically unsaturated compounds directly in water. In the prior art, small amounts of surfactants were used in the process. By xe2x80x9csurfactantsxe2x80x9d we mean organic compounds capable of forming micelles in water. These surfactants are monomer or low molecular weight derivatives which are water sensitive and can be non-ionic, anionic or cationic. Using the emulsion copolymers prepared in the presence of the surfactants in water borne coating formulations often led to problems such as poor humidity and corrosion resistance because of the presence of free surfactants which remain in the final film.
In an effort to solve these problems in the prior art, the present inventors discovered that certain structured polymers (i.e., polymers with a well defined architecture, such as AB block copolymers and graft copolymers) prepared by cobalt catalytic chain transfer polymerization act as surfactants in an emulsion copolymerization process by stabilizing the emulsion. However, unlike the surfactants used in prior art processes, the stabilizing polymers do not present disadvantages in coatings containing the emulsion and indeed can become part of the cross-linked network that forms the coating, depending on the functional groups present in the stabilizing polymer and the type of cross-linking agent used.
In the research leading up to the present invention, it was discovered that AB block copolymers prepared by the so-called Group Transfer Polymerization (xe2x80x9cGTPxe2x80x9d) process taught in Webster, U.S. Pat. No. 4,508,880, having at least 10% content in either block, were not as effective at stabilizing the emulsions. More specifically, the present inventor found that the GTP AB block copolymers with an acid content of at least 10% in either block were not able to stabilize emulsions with high solid content (i.e., greater than 25% solids), unless relatively large amounts of the GTP polymer were used. The increased amount of the GTP polymer needed to stabilize the high solids emulsions, in turn, raised the acid value of the overall emulsion to greater than 32.
Chain transfer agents employing cobalt II or III chelates such as disclosed in U.S. Pat. Nos. 4,680,352 and 4,694,054 allow the synthesis of low molecular weight (meth)acrylate based macromonomers with a terminally ethylenically unsaturated group. The term xe2x80x9cmacromonomersxe2x80x9d as understood in the art and used herein means a polymer terminating at one end with xe2x80x94Cxe2x80x94(COOR)xe2x95x90CH2. Low molecular weight oligomers prepared from such chain transfer agents act themselves as chain transfer agents for methacrylate monomers through an addition-fragmentation process allowing the synthesis of semi-block copolymers as disclosed in U.S. Pat. No. 5,371,151. The terminal unsaturated end group on the macromonomer can also co-polymerize with vinyl and acrylate type monomers to form a graft copolymer.
Block or graft copolymers can so be synthesized from a hydrophilic macromonomer via copolymerization of the macromonomer with hydrophobic co-monomers or from a hydrophobic macromonomer by copolymerization of the hydrophobic macromonomer with a hydrophilic monomer blend. By xe2x80x9chydrophilicxe2x80x9d we mean that the monomer, macromonomer or copolymer is water soluble or water dispersible. By hydrophobic we mean that the monomer, macromonomer or copolymer is not water soluble or water dispersible. Such hydrophilic macromonomers can be prepared from methacrylate monomers in the presence of a cobalt chain transfer agent. As an example, methacrylic acid monomers polymerized in the presence of the cobalt chain transfer agent can be used to form an anionic hydrophilic macromonomer which can be neutralized with a base and inverted into an aqueous solution. Similarly, a non-ionic hydrophilic macromonomer can be prepared from polyethyleneoxide derivatives of hydroxy functional methacrylates like e.g., 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate and 4-hydroxybutyl methacrylate polymerized in the presence of the cobalt chain transfer agent. And, of course, cationic macromonomers can be prepared by copolymerization of amino functional methacrylates monomers like e.g., dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, and t-butylaminoethyl methacrylate with a cobalt chain transfer agent, followed by neutralization with an acid or quaternization with e.g., alkylchloride, dialkylsulfate, or dialkylcarbonate.
The hydrophilic macromonomer is typically prepared in a solvent or solvent blend, however, it can also be prepared in water or water/solvent mixture provided the formed macromonomer is soluble in the medium. The hydrophobic macromonomer is typically prepared in a solvent blend in which it is soluble. Typical solvents that can be used to form the macromonomer and further the graft or block copolymer are alcohols as e.g., methanol, ethanol, isopropanol, n-butanol, t-butanol, isobutanol, 2-butanol, 2-ethylhexylalcohol, etc.; ketones as e.g., acetone, methylethyl ketone, isobutyl methyl ketone, diactone alcohol, etc.; esters as ethyl acetate, butyl acetate, propyl acetate, isopropyl acetate, pentyl acetate, hexyl acetate, 2-ethylhexyl acetate, etc.; glycols as ethylene glycol, propylene glycol, etc.; ethers as e.g., ethylene glycol monobutyl ether, diethylene glycol mono butyl ether, etc.; aromatic solvents as e.g., toluene, xylene, Solvesso(copyright) 100 (Exxon Chemicals), Solvesso(copyright) 150 (Exxon Chemicals), Solvesso(copyright) 200 (Exxon Chemicals), etc.
To ensure that the macromonomer only has one terminal ethylenically unsaturated group to form the graft or block copolymer, the macromonomer is polymerized by using a catalytic chain transfer agent that contains cobalt II or III such as described in U.S. Pat. Nos. 4,680,352 and 4,722,984. Most preferred are pentacyanocobaltate (ii), diaquabis(borondifluorodimethyl-glyoxymato)cobaltate (ii) and diaquabis(borondifluorophenylglyoximato) cobaltate (ii). Typically these transfer agents are used at concentration of about 5 to 5000 parts per million (xe2x80x9cppmxe2x80x9d), depending on the monomers used. The polymerization of the monomers in the presence of the cobalt chain transfer agent is done with initiators as peroxides and azo derivatives. Most preferred are azo initiators such as 2,2xe2x80x2-azobis(2-methylbutanenitrile),4,4xe2x80x2-azobis(4-cyanovaleric acid) and 2-(t-butylazo)-2-cyanopropane. The peroxide initiators include peroxyesters as e.g., t-butylperoxipivalate, t-butylperoxiacetate; peroxides as e.g., dicumyl peroxide, di-tertiary butyl peroxide, di tertiary amyl peroxide; peroxicarbonates as di(n-propyl)preoxidicarbonate and peroxi salts as ammonium peroxide. Such polymerization initiators may be activated thermally, photochemically or via redox reaction.
The polymerization process can be carried out as either batch, semi-batch, continuous or feed process at the boiling point of the solvent or below at ambient or at higher pressures. The monomer blend used in the synthesis of the hydrophilic or hydrophobic macromonomer has to contain at least 50% of methacrylate, methacrylonitrile or methacrylamide monomers or derivatives thereof.
Examples of hydrophobic methacrylate monomers are methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl methacrylate, cyclohexyl methacrylate, trimethylcyclohexyl methacrylate, isobornyl methacrylate, t-butylcyclohexyl methacrylate and benzyl methacrylate. Examples of hydrophilic methacrylate monomers include hydroxy functional monomers like 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate as well as ethoxylated or propoxylated derivatives thereof; acid functional monomers as methacrylic acid, 2-sulfoethyl methacrylate; amino functional monomers as dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, t-butylaminoethyl methacrylate, morpholinoethyl methacrylate and quaternary ammonium salts thereof. Other functional methacrylate monomers include acetoacetoxyethyl methacrylate, furfuryl methacrylate, glycidylmethacrylate, allyl methacrylate, n-(2-methacryloxyethyl)ethyleneurea, 2-cyanoethyl methacrylate and trimethoxysilylpropyl methacrylate.
Examples of methacrylonitrile and methacrylamide type monomers include alkyl or cycloalkyl methacrylamide, n-methylol methacrylamide, methoxymethyl methacrylamide, n-butoxymethyl methacrylamide, isobutoxymethyl methacrylamide, t-butylaminopropyl methacrylamide and dimethylaminopropyl methacrylamide.
The remainder of the macromonomer composition can be prepared from other polymerizable ethylinically unsaturated monomers such as alkenes, vinyl, vinylaromatic, acrylates, acrylonitrile, acrylamide and their derivatives. Examples of alkene and vinyl derivatives include dodecene, styrene, t-butylstyrene, methylstyrene, vinylacetate, vinylpropionate, versatic acid esters of vinylalcohol and vinylsilane. Examples of hydrophobic acrylates are methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, cyclohexyl acrylate, trimethylcyclohexyl acrylate, isobornyl acrylate, t-butylcyclohexyl acrylate, benzyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybuty acrylate as well as ethoxylated or propoxylated derivatives thereof; acid functional monomers as acrylic acid, itaconic acid; amino functional monomers as dimethylaminoethyl acrylate, diethylaminoethyl acrylate, t-butylaminoethyl acrylate, morpholinoethyl acrylate and quaternary ammonium salts thereof. Other functional acrylate monomers like furfuryl acrylate, glycidyl acrylate, allyl acrylate and trimethoxysilylpropyl acrylate, acrylonitrile and acrylamide and derivatives as alkyl or cycloalkyl acrylamide, n-methylol acrylamide, methoxymethyl acrylamide, n-butoxymethyl acrylamide, isobutoxymethyl acrylamide, t-butylaminopropyl acrylamide and dimethylaminopropyl acrylamide, and/or other unsaturated derivatives can be copolymerized as e.g., maleates, fumarates, maleic anhydride and acid, fumaric acid, etc. may also be used.
The macromonomer can be chemically modified before or after copolymerization by reaction of functional groups of the macromonomer with other chemicals. Acid functional groups on the macromonomer for example can be reacted with mono epoxy derivatives as butylene oxide, cyclohexeneoxide, Cardura(copyright) E10 (a C10 versatic acid glycidyl ester from Shell). Hydroxy functional groups can for examples be reacted with cyclic lactones as epsilon caprolactone. Epoxy functional groups can be modified with acid or amino functional derivatives.
The macromonomer is then further copolymerized with hydrophobic or hydrophilic monomers to form the structured (i.e., block or graft) stabilizing copolymer. A low molecular weight macromonomer will form a block stabilizing copolymer when mostly methacrylate based monomers are used in this step via an addition-fragmentation process. Acrylate and other vinyl monomers will form a graft copolymer through reaction of the terminally unsaturated group.
One portion of the structured stabilizing copolymer (i.e., either the backbone or arms of a graft copolymer or the A or B block of a block copolymer) must be hydrophilic, that is water soluble or water dispersible. Thus, at least 10% by weight of either the macromonomer or the copolymerizing co-monomers must be acid functional. Methacrylic acid is preferred, but other acid functional monomers, e.g., acrylic acid, itaconic acid, maleic acid, fumaric acid, 2-sulfoethyl methacrylate, acrylamido propane sulfonic acid, can be used to advantage. The acid functionality in the block or graft copolymer is typically neutralized with a base to make it water soluble or dispersible. Examples of bases are alkali metal (potassium, sodium, lithium) hydroxides, or ammonia, or amines. Preferred are amines as e.g. triethylamine, dimethylamino ethanol, 2-amino-2-methyl-1-propanol, 2-(dimethylamino)-2-methyl-1-propanol, diethanolamine and diisopropanolamine, for example.
The process of forming the graft or block copolymer from the macromonomer is comparable to the typical process of forming the macromonomer in which the macromonomer is used as a co-monomer. The neutralization of the acid groups of the block or graft copolymer is preferably done after the formation of the copolymer before the inversion into water. After the inversion step, the solvents used in the synthesis of the graft or block copolymer can be distilled off.
The stabilizing copolymer dispersion is then used in lieu of a surfactant in the formation of a copolymer emulsion. The amount of stabilizing copolymer used in the emulsion can be between 0.5-60% by weight, more preferably between 2-15% by weight and all ranges encompassed therein. Typical monomers used in the emulsion copolymerization step are the monomers used in the synthesis of the block copolymer. Preferably water soluble thermal initiators such as ammonium persulfate, potassium persulfate, or 4,4xe2x80x2-azo bis(4-cyano pentanoic acid) are used. Redox initiators can also be used such as e.g., t-butylhydroperoxide, cumylhydroperoxide with ascorbic acid, sodium formaldehyde sulfoxylate as reducing agent.
The surfactant free emulsions stabilized by the block or graft copolymers can be used in water borne coating compositions such as automotive primers, primer surfacers, pigmented topcoats and clear coats. Any kind of pigments used in waterborne paints such as metallic oxides like titanium dioxide, colored iron oxides, zinc oxide, talc, china clay, barium sulfate, aluminum silicates etc. and a wide variety of organic pigments such as quinacridones, phthalocyanines, perylenes, idanthrones, carbazoles and flake pigments as aluminum and pearlescent flakes may be used. The compositions can also be used in other water borne applications as inks, adhesives, cements and UV curable formulations.
The coating compositions preferably are based on (meth)acrylatexe2x80x94vinylaromatic copolymers. The copolymers can be high molecular weight to be used in only thermoplastic formulations or can be of lower molecular weight with functional groups present to be crosslinked. Examples of crosslinkable formulations are hydroxy functional copolymers crosslinked with etherified melamine, benzoguanamine or urea formaldehyde adducts. The etherifications in those crosslinkers is typically done with mono alcohols as methanol, isobutanol or n-butanol. The crosslinking can also be done with blocked or unblocked polyisocyanates. Examples of polyisocyanates are urethane adducts, biurets and cyclotrimers of hexamethylenediisocyanate, tetramethyl xylylenediisocyanate, toluenediisocyanate and isophoronediisocyanate. Examples of blocking groups are methylethyl ketoxime, caprolactam, alcohols, malonates and dimethyl pyrazole. The crosslinking can be performed at room temperature to higher temperatures up to 240xc2x0 C. Typically the curing temperature is from ambient up to 80xc2x0 C. for compositions cured with unblocked polyisocyanates while 80xc2x0 C. to 180xc2x0 C. is used for formaldehyde adducts or blocked polyisocyanates. If those crosslinkers are not water soluble or dispersible, the graft copolymer emulsion can act as emulsifier. The crosslinkers can also be hydrophilically modified to make them water soluble or dispersible. An example of hydrophilic modified crosslinkers are adducts of polyisocyanates with polyethylene glycol.
It may be desirable to add other optional ingredients to the paint formulations as antioxidants, flow modifiers, UV stabilizers, rheology control agents and/or other film forming binders can be added to the overall formulations such as binders derived from epoxy, phenolic, urethanes, polyester, polyamides, polyureas, polyacrylic or hybrids thereof.